Of the energy required to move a vessel through water, a major portion is consumed in overcoming the friction between the outer surface of the hull of the vessel and the water through which the vessel is passing. It is well known that reducing the water/hull contact area of a vessel will improve efficiency. In the past many attempts have been made to supply air or gas in the form of bubbles or streams to separate the water from the hull so as to reduce the friction there between. However, such attempts have provided little gain and in some cases loses in efficiency due to the loss of energy, in the form of the pressurized air, which flows away from the hull. The energy loss multiplies, and the efficiencies of such attempts rapidly dropped off, the deeper in the water the air is supplied, since it takes more energy/pressure to supply the air/gas at greater depths. Thus, such prior attempts were greatly limited as to the depth at which they could be effectively used. Because of the loss of efficiency at greater depths, their use has been primarily restrict to shallow draft or planning type hulls. Using reduced amounts of air/gas more effectively as micro-bubbles, or using the air in combination with textured surfaces is usually less effective in reducing friction. The efficiency of both of these methods is also limited by depth as previously discussed.
These many attempts over the years, gave up the energy stored in the form of pressurized air/gas as it exits the stern or sides and returned on its own to the surface of the water as bubbles or streams. Numerous systems and methods have been disclosed in the past using air between the outer surface of the hull of a vessel and the water to reduce friction, and therefor the energy necessary to propel the vessel through the water. The following patents disclose variations in such systems:
______________________________________ U.S. Pat. No. Inventor(s) ______________________________________ 4,340,004 Kaneil 4,393,802 Rizzo 4,523,536 Smoot 4,528,931 Lantz 4,926,771 Hull 5,031,559 Bartholomew 5,054,412 Reed et al 5,117,882 Stanford 5,176,095 Burg 5,476,056 Tokunaga et al 5,524,568 Bobst 5,575,232 Kato ______________________________________
The Kanei, Rizzo, Hull, Reed et al, and Burg patents reveal the use of channels or grooves of some sort on the external surface of the hull of a vessel, with air or gas being supplied to the channels or grooves, to reduce the surface area of contact of water with the hull of the vessel. The Hull patent shows the use of an axial supply fan 23 to supply the air. The Bartholomew, Stanford, Tokunaga et al, Bobst, and Kato et al patents disclose various means for introducing air bubbles, or an air film, over the outer surface of the hull of a ship. The remaining Smoot and Lantz patents are directed to other schemes for reducing the water drag on the hull of a ship.
While the disclosures of these patents show various ways of providing air to at least a portion of the outer surface of the hull of a vessel, to reduce the surface area of contact of water with the hull, they are all similar in that the pressurized air is permitted to escape from the hull at the sides or stern of the vessel, and presumably flow or bubble to the surface of the water in which the vessel is moving. In order to force air or any other gas to occupy a space between the hull and the water, it is necessary to provide the air or other gas at a pressure which is at least equal to, or greater than, the pressure which would be exerted by the water on the hull if the air or gas were not interposed there between. The further below the surface of the water at which the air must be supplied to the interface between the hull and the water, the greater the pressure at which it must be supplied. Thus, on fully loaded ocean going vessels, having a deep draft, air at considerable pressure must be provided to the lower portions of the hull. While providing an air interface between the outer surface of the hull and the water will reduce the friction between the water and the hull, the loss of the compressed air at the stern of the ship represents considerable lost energy. While the performance of the ship with respect to energy used to propel the ship may be improved in this way, the energy consumed in providing the compressed air, which is lost at the sides or stern of the ship, may well outweigh any improvement in performance with respect to overall energy used to propel the ship. Thus, if the loss of compressed air could be eliminated or at least reduced to a minimum, considerable less energy would be required to supply the necessary compressed air.
Accordingly, it would be advantageous to provide a method and system for reducing water friction on the hull of a marine vessel, which provides a gas, usually air, interface between the outer surface of the hull and the water in an energy efficient manner. More particularly, it would be advantageous to provide such a method and system wherein the compressed gas is recovered, usually downstream toward the stern of the vessel, thereby reducing the energy otherwise lost in permitting the compressed air escaping at the stern of the vessel.